with a small shell script, you can reproduce the wrong behaviour:

Reproduce code:
---------------

test.php:
----
<?php
exec('./test_script "test 1.jpg"',$array1,$return1);
exec('./test_script test\ 1.jpg', $array2,$return2);

echo '<pre>';
echo $return1;
print_r($array1);

echo '<br /><br />';
echo $return2;
print_r($array2);

echo '</pre>';
?>
----

test_script:
----
#!/bin/sh
echo $1
echo $2
exit 0
----


Expected result:
----------------
0Array
(
    [0] => test 1.jpg
    [1] => 
)


0Array
(
    [0] => test 1.jpg
    [1] => 
)



Actual result:
--------------

0Array
(
    [0] => test 1.jpg
    [1] => 
)


0Array
(
    [0] => test\
    [1] => 1.jpg
)

Thank you for taking the time to write to us, but this is not
a bug. Please double-check the documentation available at
http://www.php.net/manual/ and the instructions on how to report
a bug at http://bugs.php.net/how-to-report.php

In safe-mode the commands passed to exec() are always escaped. 
See http://www.php.net/manual/en/function.escapeshellcmd.php
for more information.